Tobamovirus

VIRION

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Non-enveloped, rigid helical rods with a helical symmetry. Virion is about 18 nm in diameter, and 300-310 nm in length.

GENOME

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Monopartite, linear, ssRNA(+) genome of 6.3-6.5 kb. The 3'-terminus has a tRNA-like structure. The 5' terminus has a methylated nucleotide cap (m7G5'pppG).

GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. The 5'-proximal ORFs are directly translated to produce the viral constituents of the replicase complex. RdRp is translated through suppression of termination at the end of ORF1. The small replicase is involved in replication and acts as a suppressor of RNA silencing. The movement proteins and the capsid protein are expressed from separate subgenomic mRNAs.

REPLICATION

CYTOPLASMIC

  1. Virus penetrates into the host cell.
  2. Uncoating, and release of the viral genomic RNA into the cytoplasm.
  3. The viral RNA is translated to produce the two proteins necessary for RNA synthesis (replication and transcription).
  4. Replication takes place in cytoplasmic viral factories. A dsRNA genome is synthesized from the genomic ssRNA(+).
  5. The dsRNA genome is transcribed/replicated thereby providing viral mRNAs/new ssRNA(+) genomes.
  6. The RdRp recognizes internal subgenomic promoters on the negative-sense RNA to transcribe the 3'co-terminal subgenomic RNAs that will generate the capsid protein, movement protein and viral suppressor of RNA silencing (VSR).
  7. Virus assembly in the cytoplasm. Each segment (if multipartite) is encapsidated.
  8. Viral movement protein probably mediates virion cell-to-cell transfer.

Host-virus interaction

Suppression of RNA silencing

Tobacco mosaic virus 126-kDa small replicase subunit suppresses RNA silencing .